1. Field of the Invention
Embodiments of the present invention relate to a machine tool and a control method therefor.
2. Related Art
Generally, a machine tool controls a control target such as a table on which a workpiece is mounted or a spindle head on which a tool that cuts the workpiece is mounted by using a numerical control device or a servo unit. The numerical control device outputs a position command to the servo unit. The servo unit controls the position of the control target in response to the position command via a transmission mechanism constituted by, for example, a rack and a pinion or a ball screw and a nut.
In a state where a backlash, a friction or the like is present in such a transmission mechanism, a motor is often unable to accurately control a position of the control target even if the numerical control device and the servo unit accurately control a rotational position of the motor. For example, when a backlash is present in the transmission mechanism and the servo unit is to invert the moving direction of the control target via the motor, the control target stops by as much as the backlash despite rotation (inversion) of the motor. That is, it often occurs that the control target does not instantly follow the position command. This tracking error is referred to as “lost motion”.
When a lost motion amount increases, then vibration following an axial movement becomes stronger, and a vibration pattern is transferred onto a machining surface of a machining target. This causes degradation in the quality of the machining target. Normally, a user requests a manufacturer or repair shop to repair or adjust the machine tool after noticing degradation in the machining surface. The manufacturer or repair shop sets the numerical control device and the servo unit in semi-closed loop controlled states, checks the lost motion amount using a DBB (Double Ball Bar) measuring instrument, obtains components necessary for the repair, and then repairs the machine tool. In this case, the user is unable to use the machine tool for a long period of time until the manufacturer or repair shop can obtain the components necessary for the repair.
The present invention has been achieved to solve the above problems, and an object of the invention is to provide a machine tool that is unnecessary to stop for a long period of time even if a lost motion amount is abnormally large and that can estimate an accurate lost motion amount as compared with conventional techniques, and a control method therefor.